Drosophila ananassae is a species characterized by several unusual genetic phenomena: hypermutability, meiotic male crossing over and a transposable element, tom, that often jumps causing a specific and unusual mutant phenotype, Om (dominant eye morphology). DNA sequences homologous to the tom element and the target sites of insertion have been analyzed. The tom element is a retrovirus-like transposon that is similar to the 297 element of D. melanogaster. The target site of insertion of the tom element is TATAT. The ATAT portion of the target is duplicated upon insertion. Many other features of typical retroviruses are found in the structure of the tom element. The analysis of a set of Om mutants at particular locus, Om(1D), indicates that the mutant phenotype is associated with the insertion of the tom element(s) in a number of sites in a region greater than 25 kilobases. Spontaneous revertants are due to the excision of tom elements.